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NR-U’s definition of success for LBT needs to be realigned with 802.11 & European rules

This article discusses the need to realign NR-U's definition of success for Listen Before Talk (LBT) with the rules of 802.11 and European regulations. It emphasizes on the importance of focusing on the start of transmission to enable compliance and fairness. The article also addresses the inconsistencies between LAA/NR-U and European rules regarding CW updates.

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NR-U’s definition of success for LBT needs to be realigned with 802.11 & European rules

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  1. NR-U’s definition of success for LBT needs to be realigned with 802.11 & European rules • 1 July 2019 Authors: Andrew Myles, Cisco

  2. NR-U’s definition of success for LBT needs to be realigned with 802.11 and European rules • Executive Summary • European rules & IEEE 802.11 base Contention Window (CW) updates in LBT on the success of transmissions at the start of a COT … • … because only the success (or lack thereof) at the start of COT is relevant to collision detection for LBT • Whereas, LAA/NR-U bases Contention Window(CW) updates on information later in the COT, contrary to the European rules • It is time to align NR-U to focus success on start of transmission to enable compliance & fairness Cisco

  3. European rules & IEEE 802.11 base CW updates in LBT on the success of tx’s at the start of a COT … • The European 5 GHz rules (as currently documented in EN 301 893 v2.1.1) for updating the LBT Contention Window (CW) require … • … atleast one transmission that started at the beginning of the Channel Occupancyneeds to be successful before CW can be reset, otherwise CW is doubled • The CW update rules in for EDCA based access in IEEE 802.11 are consistent with these European rules • IEEE 802.11 generally focuses on start of a TXOP to determine success or otherwise for the purposes of CW updates • There are some interesting exceptions in the IEEE 802.11 standard, such as broadcast frames and delayed block acks • However, they generally do not matter in practice; see BRAN(19)102015 for more details Andrew Myles, Cisco

  4. … because only the success (or lack thereof) at the start of COT is relevant to collision detection for LBT Errors at the start of a COT indicate a collision for LBT; no errors at the start indicate a lack of a collision COT/TXOP 1 Errors later in a COT are not indicative of a collision Collision COT/TXOP 2 Note: An implicit assumption is that the start of transmissions align, which is true if one assumes successful use of hidden station mitigation mechanisms Andrew Myles, Cisco

  5. Whereas, LAA/NR-U bases CW updates on information later in COT, contrary to the European rules • It is understood that LAA/NR-U does not focus its evaluation of success at the start, possibly making it non-compliant with the European rules • Rather, it evaluates the fraction of the COT that was rx’ed correctly to determine success, which may not include the start of the COT • The situation is even worse if there is significant use of reservation signals at the start of a COT because this period is not evaluated at all • The result is unfair access to LAA/NR-U, relative to Wi-Fi! Wi-Fi detects collision If 80% correct reception is threshold for success then this case might not be detected as a collision by LAA/NR-U; it should be! Wi-Fi TXOP Reservation signal NR-U COT Andrew Myles, Cisco

  6. It is time to align NR-U to focus success on start of transmission to enable compliance & fairness • Now would be a great time to fix NR-U so that collision detection focuses on the start of the COT • The fix may require significant changes to the NR-U specification • It may be less important to fix LAA, depending on one’s view of its market importance • A fix is important to: • Avoid future compliance issues for NR-U in Europe ... • … and more importantly to promote fair access in a shared environment by aligning NR-U with the spirit of LBT based coexistence Andrew Myles, Cisco

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